[linux-2.6-block.git] / Documentation / i2c / functionality.rst

=======================
I2C/SMBus Functionality
=======================

INTRODUCTION
------------

Because not every I2C or SMBus adapter implements everything in the
I2C specifications, a client can not trust that everything it needs
is implemented when it is given the option to attach to an adapter:
the client needs some way to check whether an adapter has the needed
functionality.


FUNCTIONALITY CONSTANTS
-----------------------

For the most up-to-date list of functionality constants, please check
<uapi/linux/i2c.h>!

  =============================== ==============================================
  I2C_FUNC_I2C                    Plain i2c-level commands (Pure SMBus
                                  adapters typically can not do these)
  I2C_FUNC_10BIT_ADDR             Handles the 10-bit address extensions
  I2C_FUNC_PROTOCOL_MANGLING      Knows about the I2C_M_IGNORE_NAK,
                                  I2C_M_REV_DIR_ADDR and I2C_M_NO_RD_ACK
                                  flags (which modify the I2C protocol!)
  I2C_FUNC_NOSTART                Can skip repeated start sequence
  I2C_FUNC_SMBUS_QUICK            Handles the SMBus write_quick command
  I2C_FUNC_SMBUS_READ_BYTE        Handles the SMBus read_byte command
  I2C_FUNC_SMBUS_WRITE_BYTE       Handles the SMBus write_byte command
  I2C_FUNC_SMBUS_READ_BYTE_DATA   Handles the SMBus read_byte_data command
  I2C_FUNC_SMBUS_WRITE_BYTE_DATA  Handles the SMBus write_byte_data command
  I2C_FUNC_SMBUS_READ_WORD_DATA   Handles the SMBus read_word_data command
  I2C_FUNC_SMBUS_WRITE_WORD_DATA  Handles the SMBus write_byte_data command
  I2C_FUNC_SMBUS_PROC_CALL        Handles the SMBus process_call command
  I2C_FUNC_SMBUS_READ_BLOCK_DATA  Handles the SMBus read_block_data command
  I2C_FUNC_SMBUS_WRITE_BLOCK_DATA Handles the SMBus write_block_data command
  I2C_FUNC_SMBUS_READ_I2C_BLOCK   Handles the SMBus read_i2c_block_data command
  I2C_FUNC_SMBUS_WRITE_I2C_BLOCK  Handles the SMBus write_i2c_block_data command
  =============================== ==============================================

A few combinations of the above flags are also defined for your convenience:

  =========================       ======================================
  I2C_FUNC_SMBUS_BYTE             Handles the SMBus read_byte
                                  and write_byte commands
  I2C_FUNC_SMBUS_BYTE_DATA        Handles the SMBus read_byte_data
                                  and write_byte_data commands
  I2C_FUNC_SMBUS_WORD_DATA        Handles the SMBus read_word_data
                                  and write_word_data commands
  I2C_FUNC_SMBUS_BLOCK_DATA       Handles the SMBus read_block_data
                                  and write_block_data commands
  I2C_FUNC_SMBUS_I2C_BLOCK        Handles the SMBus read_i2c_block_data
                                  and write_i2c_block_data commands
  I2C_FUNC_SMBUS_EMUL             Handles all SMBus commands that can be
                                  emulated by a real I2C adapter (using
                                  the transparent emulation layer)
  =========================       ======================================

In kernel versions prior to 3.5 I2C_FUNC_NOSTART was implemented as
part of I2C_FUNC_PROTOCOL_MANGLING.


ADAPTER IMPLEMENTATION
----------------------

When you write a new adapter driver, you will have to implement a
function callback ``functionality``. Typical implementations are given
below.

A typical SMBus-only adapter would list all the SMBus transactions it
supports. This example comes from the i2c-piix4 driver::

  static u32 piix4_func(struct i2c_adapter *adapter)
  {
	return I2C_FUNC_SMBUS_QUICK | I2C_FUNC_SMBUS_BYTE |
	       I2C_FUNC_SMBUS_BYTE_DATA | I2C_FUNC_SMBUS_WORD_DATA |
	       I2C_FUNC_SMBUS_BLOCK_DATA;
  }

A typical full-I2C adapter would use the following (from the i2c-pxa
driver)::

  static u32 i2c_pxa_functionality(struct i2c_adapter *adap)
  {
	return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL;
  }

I2C_FUNC_SMBUS_EMUL includes all the SMBus transactions (with the
addition of I2C block transactions) which i2c-core can emulate using
I2C_FUNC_I2C without any help from the adapter driver. The idea is
to let the client drivers check for the support of SMBus functions
without having to care whether the said functions are implemented in
hardware by the adapter, or emulated in software by i2c-core on top
of an I2C adapter.


CLIENT CHECKING
---------------

Before a client tries to attach to an adapter, or even do tests to check
whether one of the devices it supports is present on an adapter, it should
check whether the needed functionality is present. The typical way to do
this is (from the lm75 driver)::

  static int lm75_detect(...)
  {
	(...)
	if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA |
				     I2C_FUNC_SMBUS_WORD_DATA))
		goto exit;
	(...)
  }

Here, the lm75 driver checks if the adapter can do both SMBus byte data
and SMBus word data transactions. If not, then the driver won't work on
this adapter and there's no point in going on. If the check above is
successful, then the driver knows that it can call the following
functions: i2c_smbus_read_byte_data(), i2c_smbus_write_byte_data(),
i2c_smbus_read_word_data() and i2c_smbus_write_word_data(). As a rule of
thumb, the functionality constants you test for with
i2c_check_functionality() should match exactly the i2c_smbus_* functions
which you driver is calling.

Note that the check above doesn't tell whether the functionalities are
implemented in hardware by the underlying adapter or emulated in
software by i2c-core. Client drivers don't have to care about this, as
i2c-core will transparently implement SMBus transactions on top of I2C
adapters.


CHECKING THROUGH /DEV
---------------------

If you try to access an adapter from a userspace program, you will have
to use the /dev interface. You will still have to check whether the
functionality you need is supported, of course. This is done using
the I2C_FUNCS ioctl. An example, adapted from the i2cdetect program, is
below::

  int file;
  if (file = open("/dev/i2c-0", O_RDWR) < 0) {
	/* Some kind of error handling */
	exit(1);
  }
  if (ioctl(file, I2C_FUNCS, &funcs) < 0) {
	/* Some kind of error handling */
	exit(1);
  }
  if (!(funcs & I2C_FUNC_SMBUS_QUICK)) {
	/* Oops, the needed functionality (SMBus write_quick function) is
           not available! */
	exit(1);
  }
  /* Now it is safe to use the SMBus write_quick command */
Commit	Line	Data
ccf988b6 MCC	1	=======================
	2	I2C/SMBus Functionality
	3	=======================
	4
1da177e4 LT	5	INTRODUCTION
	6	------------
	7
ccf988b6	8	Because not every I2C or SMBus adapter implements everything in the
1da177e4 LT	9	I2C specifications, a client can not trust that everything it needs
	10	is implemented when it is given the option to attach to an adapter:
	11	the client needs some way to check whether an adapter has the needed
ccf988b6	12	functionality.
1da177e4 LT	13
	14
	15	FUNCTIONALITY CONSTANTS
	16	-----------------------
	17
	18	For the most up-to-date list of functionality constants, please check
8421d4fe	19	<uapi/linux/i2c.h>!
1da177e4	20
ccf988b6	21	=============================== ==============================================
1da177e4 LT	22	I2C_FUNC_I2C Plain i2c-level commands (Pure SMBus
	23	adapters typically can not do these)
	24	I2C_FUNC_10BIT_ADDR Handles the 10-bit address extensions
d057c96c	25	I2C_FUNC_PROTOCOL_MANGLING Knows about the I2C_M_IGNORE_NAK,
14674e70 MB	26	I2C_M_REV_DIR_ADDR and I2C_M_NO_RD_ACK
	27	flags (which modify the I2C protocol!)
	28	I2C_FUNC_NOSTART Can skip repeated start sequence
1da177e4 LT	29	I2C_FUNC_SMBUS_QUICK Handles the SMBus write_quick command
	30	I2C_FUNC_SMBUS_READ_BYTE Handles the SMBus read_byte command
	31	I2C_FUNC_SMBUS_WRITE_BYTE Handles the SMBus write_byte command
	32	I2C_FUNC_SMBUS_READ_BYTE_DATA Handles the SMBus read_byte_data command
	33	I2C_FUNC_SMBUS_WRITE_BYTE_DATA Handles the SMBus write_byte_data command
	34	I2C_FUNC_SMBUS_READ_WORD_DATA Handles the SMBus read_word_data command
	35	I2C_FUNC_SMBUS_WRITE_WORD_DATA Handles the SMBus write_byte_data command
	36	I2C_FUNC_SMBUS_PROC_CALL Handles the SMBus process_call command
	37	I2C_FUNC_SMBUS_READ_BLOCK_DATA Handles the SMBus read_block_data command
	38	I2C_FUNC_SMBUS_WRITE_BLOCK_DATA Handles the SMBus write_block_data command
	39	I2C_FUNC_SMBUS_READ_I2C_BLOCK Handles the SMBus read_i2c_block_data command
	40	I2C_FUNC_SMBUS_WRITE_I2C_BLOCK Handles the SMBus write_i2c_block_data command
ccf988b6	41	=============================== ==============================================
1da177e4 LT	42
	43	A few combinations of the above flags are also defined for your convenience:
	44
ccf988b6	45	========================= ======================================
1da177e4 LT	46	I2C_FUNC_SMBUS_BYTE Handles the SMBus read_byte
	47	and write_byte commands
	48	I2C_FUNC_SMBUS_BYTE_DATA Handles the SMBus read_byte_data
	49	and write_byte_data commands
	50	I2C_FUNC_SMBUS_WORD_DATA Handles the SMBus read_word_data
	51	and write_word_data commands
	52	I2C_FUNC_SMBUS_BLOCK_DATA Handles the SMBus read_block_data
	53	and write_block_data commands
	54	I2C_FUNC_SMBUS_I2C_BLOCK Handles the SMBus read_i2c_block_data
	55	and write_i2c_block_data commands
6808b002	56	I2C_FUNC_SMBUS_EMUL Handles all SMBus commands that can be
1da177e4 LT	57	emulated by a real I2C adapter (using
1da177e4 LT	58	the transparent emulation layer)
ccf988b6	59	========================= ======================================
1da177e4	60
14674e70 MB	61	In kernel versions prior to 3.5 I2C_FUNC_NOSTART was implemented as
	62	part of I2C_FUNC_PROTOCOL_MANGLING.
	63
1da177e4	64
88b28328 JD	65	ADAPTER IMPLEMENTATION
88b28328 JD	66	----------------------
1da177e4	67
88b28328	68	When you write a new adapter driver, you will have to implement a
ccf988b6	69	function callback ``functionality``. Typical implementations are given
88b28328	70	below.
1da177e4	71
88b28328	72	A typical SMBus-only adapter would list all the SMBus transactions it
ccf988b6	73	supports. This example comes from the i2c-piix4 driver::
88b28328 JD	74
	75	static u32 piix4_func(struct i2c_adapter *adapter)
	76	{
	77	return I2C_FUNC_SMBUS_QUICK \| I2C_FUNC_SMBUS_BYTE \|
	78	I2C_FUNC_SMBUS_BYTE_DATA \| I2C_FUNC_SMBUS_WORD_DATA \|
	79	I2C_FUNC_SMBUS_BLOCK_DATA;
1da177e4 LT	80	}
1da177e4 LT	81
88b28328	82	A typical full-I2C adapter would use the following (from the i2c-pxa
ccf988b6	83	driver)::
1da177e4	84
88b28328	85	static u32 i2c_pxa_functionality(struct i2c_adapter *adap)
1da177e4	86	{
88b28328	87	return I2C_FUNC_I2C \| I2C_FUNC_SMBUS_EMUL;
1da177e4 LT	88	}
1da177e4 LT	89
88b28328 JD	90	I2C_FUNC_SMBUS_EMUL includes all the SMBus transactions (with the
	91	addition of I2C block transactions) which i2c-core can emulate using
	92	I2C_FUNC_I2C without any help from the adapter driver. The idea is
	93	to let the client drivers check for the support of SMBus functions
	94	without having to care whether the said functions are implemented in
	95	hardware by the adapter, or emulated in software by i2c-core on top
	96	of an I2C adapter.
1da177e4 LT	97
	98
	99	CLIENT CHECKING
	100	---------------
	101
	102	Before a client tries to attach to an adapter, or even do tests to check
	103	whether one of the devices it supports is present on an adapter, it should
88b28328	104	check whether the needed functionality is present. The typical way to do
ccf988b6	105	this is (from the lm75 driver)::
1da177e4	106
88b28328	107	static int lm75_detect(...)
1da177e4	108	{
88b28328 JD	109	(...)
	110	if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA \|
	111	I2C_FUNC_SMBUS_WORD_DATA))
	112	goto exit;
	113	(...)
1da177e4 LT	114	}
1da177e4 LT	115
88b28328 JD	116	Here, the lm75 driver checks if the adapter can do both SMBus byte data
	117	and SMBus word data transactions. If not, then the driver won't work on
	118	this adapter and there's no point in going on. If the check above is
	119	successful, then the driver knows that it can call the following
	120	functions: i2c_smbus_read_byte_data(), i2c_smbus_write_byte_data(),
	121	i2c_smbus_read_word_data() and i2c_smbus_write_word_data(). As a rule of
	122	thumb, the functionality constants you test for with
	123	i2c_check_functionality() should match exactly the i2c_smbus_* functions
	124	which you driver is calling.
	125
	126	Note that the check above doesn't tell whether the functionalities are
	127	implemented in hardware by the underlying adapter or emulated in
	128	software by i2c-core. Client drivers don't have to care about this, as
	129	i2c-core will transparently implement SMBus transactions on top of I2C
	130	adapters.
1da177e4 LT	131
	132
	133	CHECKING THROUGH /DEV
	134	---------------------
	135
	136	If you try to access an adapter from a userspace program, you will have
	137	to use the /dev interface. You will still have to check whether the
	138	functionality you need is supported, of course. This is done using
88b28328	139	the I2C_FUNCS ioctl. An example, adapted from the i2cdetect program, is
ccf988b6	140	below::
1da177e4 LT	141
1da177e4 LT	142	int file;
88b28328	143	if (file = open("/dev/i2c-0", O_RDWR) < 0) {
1da177e4 LT	144	/* Some kind of error handling */
	145	exit(1);
	146	}
88b28328	147	if (ioctl(file, I2C_FUNCS, &funcs) < 0) {
1da177e4 LT	148	/* Some kind of error handling */
	149	exit(1);
	150	}
88b28328	151	if (!(funcs & I2C_FUNC_SMBUS_QUICK)) {
1da177e4 LT	152	/* Oops, the needed functionality (SMBus write_quick function) is
	153	not available! */
	154	exit(1);
	155	}
	156	/* Now it is safe to use the SMBus write_quick command */